As a nickel electrode made up by directly loading a nickel hydroxide powder forming an active material, there are a paste-type electrode in which the nickel hydroxide powder is formed into a paste and loaded in a porous substrate comprising a nickel, and a button-type electrode and a pocket-type electrode in which the nickel hydroxide powder is wrapped by a current collector. The porous substrate comprising an alkaline-proof metal forming the current corrector is comparatively expensive in the paste-type electrode, so that a study has been made on an electrode utilizing a core metal as its current collector.
On the other hand, with a recent tendency to minimize a size and weight of portable electric equipment, a demand for enhancing an energy density is increasing against batteries serving as portable power sources. In order to meet this demand, a nickel-cadmium battery, a nickel-metal hydride battery and a nickel-zinc battery etc., which utilize a paste-type nickel electrode, have been developed and put to practical use.
The paste-type nickel electrode is made up by forming the nickel hydroxide powder into a paste and directly loading it in a porous substrate comprising a nickel. Therefore, in order to obtain a paste-type nickel electrode having a high energy density, it is enough to increase a porosity of the electrode substrate and a density of the nickel hydroxide powder forming the active material.
A nickel fibrous porous body and a foamed nickel porous body etc. which have a porosity of about 95% which is approximately a manufacture limit of porosity have been put in practical use for the electrode substrate. A nickel hydroxide powder an inner pore volume of which is controlled to under 0.1 ml/g incl. has been developed for the active material. Since the nickel hydroxide powder has a bulk density as high as about 2 g/ml, it becomes possible to load the powder by about 20% more than conventional one. By using the high-porous electrode substrate and the high-density nickel hydroxide powder, it becomes possible to make up a paste-type nickel electrode having 550 mAh/ml or larger so that the electrode is applied to a battery system having a high energy density.
On the other hand, the nickel hydroxide powder the inner pore volume of which is controlled to under 0.1 ml/g incl. has a feature that it is obtained in a spherical shape caused by its manufacturing process. For this reason, clearances are produced between powder particles under a loaded state as illustrated by FIG. 7, and this point has been a negative primary factor to increase the energy density.